A multifocal lens is constituted of a principal lens in glass and of several segments in glass which are fused onto it. These segments have a higher refractive index than that of the principal lens; this enables obtaining areas of higher corrective power. Said segment(s) is (are) normally soldered onto the principal lens during a heat treatment bringing the whole of the glasses to a temperature close to the softening point of the glass constituting the principal lens. Consequently, the segment glasses must have characteristics of viscosity close to those of the glass of the principal lens. It must also have close thermal dilation characteristics, so that the resulting soldering has a very low strain level.
The objective of the present invention was to obtain glasses of refractive index between 1.70 and 1.78, fusible particularly to an ophthalmic glass manufactured by the Applicant Company (Corning S. A. ), of index 1.6, of Abbe number 41 and which has the following physical characteristics:
thermal dilation coefficient: 100.times.10.sup.-7 K.sup.-1 (between 25 and 300.degree. C.), PA1 softening point: 610.degree. C., PA1 strain point: 460.degree. C., PA1 annealing point: 490.degree. C. PA1 high refractive index: 1.70.ltoreq.n.ltoreq.1.78; PA1 thermal dilation coefficient: between 90 and 110.times.10.sup.-7 K.sup.-1 (between 25 and 300.degree. C.); PA1 softening point: between about 570 and 620.degree. C. PA1 U.S. Pat. No. 4,568,651 (HOYA) describing glasses having the following composition of oxides expressed in percentages by weight: PA1 the Japanese patent JP-A-61 146730 (NIPPON KOGAKU KK) describing glasses having the following composition of oxides, expressed in molar percentages: PA1 for the US patent, glasses having a refractive index of 1.73 to 1.78. The dilation and viscosity of said glasses are not specified. These glasses have contents in PbO and, in TiO.sub.2 and in alkalis, respectively much higher and much lower than those of the glasses of the invention; PA1 for the JP patents, glasses having refractive indices and softening points close to those of the glasses of the invention. The thermal dilation coefficients of said glasses are not specified. The composition ranges of said glasses are very different to that claimed here: the glasses in particular contain neither any PbO, nor any TiO.sub.2. PA1 thermal dilation coefficient: 100.times.10.sup.-7 K.sup.-1 (between 25 and 300.degree. C.); PA1 softening point: 610.degree. C.; PA1 refractive index: 1.6.
Said glass of the Applicant Company bears the Code-name 8008. It was described in U.S. Pat. No. 4,540,672.
It has been determined that, in order to be fusible to said glass Code-named 8008, the glasses of the invention must have a thermal dilation coefficient between about 90 and 110.times.10.sup.-7 K.sup.-1 and a softening point between about 570 and 620.degree. C.
The technical problem to be solved within the context of the present invention has therefore been that of providing glasses having the specifications below: